dt-bindings:iio:humidity:htu21 Drop separate doc + add to trivial-devices

Very simple binding for this i2c device with no properties beyond
reg and compatible.  Hence doesn't need it's own document.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031134110.724233-3-jic23@kernel.org